1. Field of the Invention
The invention relates to high pressure discharge lamps, and more specifically, to high pressure discharge lamps having an electrical element in series with a discharge vessel and to the mount construction for mounting the discharge vessel within an outer envelope in such lamps. The invention also relates to a simplified arrangement for mounting the components of a starting circuit and a series resistor to the lamp frame.
2. Description of the Prior Art
High pressure sodium (HPS) discharge lamps typically have an elongate tubular ceramic discharge vessel, or arc tube, and a pair of conductive feed-throughs which extend through opposite ends of the arc tube and are connected to respective discharge electrodes within the arc tube. "Ceramic" as used herein means a crystalline oxide resistant to sodium, for example monocrystalline sapphire or polycrystalline densely-sintered aluminum oxide. A frame within an outer envelope supports the arc tube and applies an electric potential across the feed-throughs for energizing the arc tube to emit light. As known from U.S. Pat. Nos. 4,498,030; 4,328,445; 4,297,725; and 4,144,475, a desirable frame includes a rigid current-conductor extending from the lamp stem which supports and is electrically connected to the arc tube feed-through proximate the lamp stem. A second rigid current-conductor extends the length of the discharge vessel and supports and electrically connects the feed-through remote from the lamp stem. The above frames are desirable because they provide reliable support and electrical connection to the arc tube with a minimum number of parts.
For HPS lamps intended for operation on ballasts without a starter it is desirable to include a starting circuit within the envelope for starting the arc tube. A common starting circuit includes a glow starter in series with a current limiting resistor and a bimetal switch, all of which are electrically in parallel with the arc tube. The starting circuit generates a voltage, typically several kilovolts, sufficient for inducing ionization throughout the discharge vessel. Typically, the resistors used in HPS starting circuits dissipate greater than 100 watts during lamp starting, but are electrically disconnected from the lamp circuit by the glow starter upon ignition of the discharge arc. Within several minutes after ignition of the discharge arc, heat from the discharge vessel causes the bimetal switch to open and physically and electrically disconnect the glow starter and the starting resistor from the discharge vessel circuit.
Filament resistors have been used in starting circuits but have the disadvantage that they are generally long, and as a result are formed into coils and/or suspended in zig-zag form, causing space and mounting problems within the lamp envelope. More recently, ceramic thick film resistors, wherein a thick film resistive element such as tungsten is disposed on an insulative substrate, have also been used in starting circuits for HID lamps as is known from U.S. Pat. No. 5,008,583 (Carlet) and Japanese Kokai 56-73856.
HPS lamps, as with other discharge lamps, may have an electrical element in series with the arc tube and mounted within the outer envelope to alter or improve lamp performance. Recently, unsaturated HPS lamps have been used as retrofit lamps to replace mercury vapor lamps operated on constant-wattage type ballasts, for example, CWA ballasts. However, when operated on constant wattage ballasts, unsaturated HPS lamps of certain wattages may under certain conditions exhibit flicker of the discharge arc, especially for ballasts operating near or outside their rated specification. To eliminate flicker, it is known to provide a low ohmic value resistor in series with the unsaturated arc tube. Japanese Kokai 1-211896 shows an unsaturated HPS lamp having a series resistor which reduces the reignition arc voltage of the arc tube to prevent flicker of the arc. However, in JP Kokai 1-211896, the series resistor is a filament resistor mounted the lamp frame at the dome end of the lamp and connected to the lead-through remote from the lamp stem. This position is undesirable because it causes shadowing of light emitted from the discharge vessel and also requires a complex construction. U.S. Pat. No. 4,258,288 (Michael et al) shows a resistor in series with a metal halide arc tube which series resistor is also connected to the lead-through remote from the lamp stem and mounted at the dome end of the lamp envelope.
It would be desirable to mount the series resistor adjacent the reentrant stem of an unsaturated HPS lamp which also includes a starting circuit within the lamp envelope for operation on a CWA ballast. However, mounting of the starting resistor, glow starter, and bimetal switch near the base end of the lamp envelope is space consuming and typically requires multiple welds to the lamp frame. Mounting of an additional resistor component on the lamp frame between the discharge vessel and the lamp stem has not been practicable. For mercury-retrofit HPS lamps, the light center length of the arc tube measured from the base should equal the light center length of the mercury vapor lamp which it replaces to obtain optimum optical performance in the luminaire. Thus, it is not feasible to position the arc tube further from the base to obtain more mounting space on the lamp frame.
The above mentioned copending application to J. Ravi claims a thick film resistor which includes a starting resistor and a flicker elimination series resistor on an integral substrate. The use of such a combination resistor alleviates some of the space problems associated with mounting multiple components on the lamp frame adjacent the lamps stem. However, since in the desirable mount construction discussed above the shorter current-conductor provides electrical connection to the feed-through proximate the base, a series resistor cannot be connected between the shorter current-conductor and the feed-through with such a mount.
Accordingly, it is an object of the invention to provide an improved mount construction for a high pressure discharge lamp having an electrical element in series with the discharge vessel to lower the cost and increase the reliability of the lamp.
Another object of the invention is to provide a mount construction which facilitates the mounting of a series resistor substantially axially between the discharge vessel and the lamp stem along with the components of a starting circuit.
Yet another object of the invention is to provide an improved mount construction for a high pressure sodium discharge lamp which facilitates the electrical connection of a series resistor and the components of a starting circuit.